1. Field of the Invention
The present invention relates to a laser irradiation apparatus utilized for crystallizing a semiconductor film. Moreover, the present invention relates to a laser irradiation method and a method for manufacturing a semiconductor device using the laser irradiation apparatus of the present invention.
2. Description of the Related Art
Thin film transistor employing poly-crystalline semiconductor film (poly-crystalline TFT) is superior by double digits or more to TFT employing an amorphous semiconductor film in terms of its mobility, and thereby has an advantage that a pixel portion and a peripheral driver circuit thereof in a semiconductor display device can be integrated on a same substrate. The poly-crystalline semiconductor film can be formed over an inexpensive glass substrate by employing a laser annealing method.
Laser oscillators are generally classified into two types of pulsed laser oscillators and continuous wave (CW) laser oscillators. The output energy of the pulsed laser oscillators, typically excimer laser, is higher than that of the CW laser oscillators by triple to six digits. Therefore, throughput can be enhanced by shaping a beam spot (a region in which the laser beam is irradiated in fact to the surface of a processing object) into square with several centimeters on a side or linear with not less than 100 mm in length through an optical system and irradiating the laser beam to the semiconductor film effectively. As a result, the pulsed laser oscillators have become popular to be employed for the crystallization of the semiconductor film.
It is noted that “linear” here does not mean a line strictly but means a rectangle (or an oblong) with a large aspect ratio. For example, the expression of “linear” indicates a rectangle with an aspect ratio of two or more (preferably, 10 to 10000), which is still included in a beam spot that is rectangular in shape on the surface of the processing object.
However, the semiconductor film crystallized by using a pulsed laser beam as described above comprises a plurality of crystal grains assembled and the position and the size of the crystal grain are random. Compared to an inside of the crystal grain, a boundary between the crystal grains (crystal grain boundary) has an amorphous structure and an infinite number of recombination centers and trapping centers existing due to crystal defects. It is a problem that when a carrier is trapped in the trapping center, potential of the crystal grain boundary increases to become a barrier against the carrier, and thereby deteriorating a carrier mobility.
In view of such problem, recently, attention has been paid to the technique of irradiating a continuous wave (CW) laser beam to a semiconductor film. In this technique, the CW laser beam is scanned to one direction to grow crystals continuously toward the scanning direction so as to form a plurality of crystal grains comprising single-crystal grains extending long in the direction thereof. It is considered that this technique enables to form a TFT that has almost no crystal grain boundary at least in a channel direction of the TFT.
By the way, it is preferable that the absorption coefficient of the laser beam to the semiconductor film is high in order to crystallize the semiconductor film more effectively. The absorption coefficient to the semiconductor film depends on the material and the like. In case of using a YAG laser or a YVO4 laser to crystallize the silicon film having a thickness of several tens to several hundreds nm which is generally employed for the semiconductor device, the second harmonic which has a shorter wavelength than the fundamental wave is higher in absorption coefficient, and thereby crystallization can be more effective.
However, the energy of the laser beam converted into the second harmonic is lower than that of the fundamental wave. Therefore it is difficult to enhance throughput by enlarging the area of the beam spot. Especially, since the output energy from the CW laser oscillator per unit time is lower than that from the pulsed laser oscillator, throughput is difficult to be enhanced. For example, when a Nd: YAG laser is used, the conversion efficiency from the fundamental wave (wavelength 1064 nm) to the second harmonic (wavelength 532 nm) is about 50%. Moreover, the non-linear optical element which converts the laser beam into the second harmonic does not have enough resistance against the laser beam. For example, the CW YAG laser can output the fundamental wave for 10 kW, while the second harmonic for 10 W. Therefore, in order to obtain necessary energy density for crystallizing the semiconductor film, the area of the beam spot must be narrowed for about 10−3 mm2, and thereby the CW YAG laser is inferior to the pulsed excimer laser in terms of throughput.
It is noted that in both ends of the beam spot in the direction perpendicular to the scanning direction, there is formed a region where the crystal grain is extremely small and inferior to the center of the beam spot in its crystallinity. Even though a semiconductor element is formed in such a region, a high characteristic cannot be expected. Therefore, it is important to reduce the proportion of the region where the crystallinity is inferior among the regions where the laser beam is irradiated in order to ease the restriction in the layout of the semiconductor element.